Membrane separation processes have been rapidly expanding in recent years. Composite membranes are typically made of a porous substrate covered with a membrane layer. The porous substrate provides mechanical strength without significantly reducing the flux through the membranes. Ceramic membranes made of γ-alumina have ion retention characteristics and, as such, have been used in various aqueous and gas separation applications. For example, γ-alumina is known to have a significant affinity for carbon dioxide (CO2), which makes γ-alumina membranes suitable for carbon dioxide separation applications, such as, for example, steam reforming of hydrocarbons, coal gasification, partial oxidation of natural gas, and/or biomass gasification. However, the gas separation functions of γ-alumina membranes have not been well demonstrated because making a defect-free membrane with a predetermined structure remains a large material processing challenge. Therefore, new metallic-ceramic composite membranes and a new method for producing metallic-ceramic composite membranes using γ-alumina are needed.